1. Field of the Invention
The present invention relates to a procedure and a device for data transmission between a processing unit and a plurality of position measuring instruments that are joined together in a linear bus topology.
2. Discussion of Related Art
From German Patent Disclosure DE 100 30 358 A1 of the present applicant, both a procedure and a device for serial data transmission between a processing unit and a position measuring instrument are known. Via a digital interface, binary data words in a continuous data stream over two data channels are exchanged between the processing unit and the position measuring instrument. In conjunction with this digital interface, German Patent Disclosure DE 100 30 357 A1 should also be mentioned, in which certain provisions are proposed that are of significance in particular with a view to the most time-determined possible acquisition of position data and storage of position data in the position measuring instrument.
The concept of a digital, serial interface disclosed in these references is in principle not limited to a particular interface physics but can instead be realized in combination with various variants and embodiments of known interface technologies.
In the references, only a so-called point-to-point connection between a single position measuring instrument and a central processing unit—such as a numerical control for a machine tool—is disclosed explicitly. In principle, the provisions described can also be employed with so-called bus topology, however, in which one processing unit is connected to a plurality of position measuring instruments. When high data transmission rates are required, the bus topology provided can in particular be a linear bus topology, in which the individual bus units are connected to one another via individual point-to-point connections. In the direction of the processing unit, data are exchanged between the various bus units by the bucket-chain principle. In this context, the term “bucket-chain principle” is understood to mean the transmission of data from one bus unit to the next bus unit.
In practice, in such bus topologies, significant and variable signal transit times result between the various position measuring instruments and the processing unit. The signal transit times can be ascribed here to the requisite transit times in the connecting lines and the requisite processing times in the various position measuring instruments through which the data must pass in the course of the transmission, because of the aforementioned transmission principle.
Unless additional provisions are made, time- determined—that is, simultaneous—acquisition of measured values by all the position measuring instruments cannot be achieved. For precise further processing of the position data, however, a fundamental prerequisite is the defined or as simultaneous as possible acquisition of measured values and execution of position data request signals.